WASHINGTON/MANILA U.S. President Barack Obama sought to reassure allies in Asia last month that the United States would support them in the face of a more assertive China.

But after one of Beijing's boldest moves in years to lay claim to contested waters off Vietnam, some Asia countries are asking a simple question: Where is Washington?

Days after Obama left the region, China deployed an oil drilling rig 150 miles off the coast of Vietnam, into a part of the South China Sea claimed by itself and Hanoi. That sparked deadly anti-China riots in central Vietnam and raised questions over whether Obama's long-promised strategic "pivot" of military assets to Asia is more than talk.

"We have been pushing the U.S. to change its policy and take sides in the regional dispute," said a senior Philippine defense official. "I wanted to see the U.S. match with stronger action what President Obama has said during his recent visit in the Philippines."

China claims a stretch of water off its south coast and to the east of mainland Southeast Asia, setting it in direct conflict with claims of U.S. allies Vietnam and the Philippines. Brunei, Taiwan and Malaysia also lay claim to parts of the South China Sea.

At stake is control over what are believed to be significant reserves of oil and gas. Estimates for proven and undiscovered oil reserves in the entire sea range from 28 billion to as high as 213 billion barrels of oil, the U.S. Energy Information Administration said in a March 2008 report.

The United States has responded with sharpened rhetoric toward China, describing a pattern of Chinese "provocations".

But it has taken no tangible action, a stark reflection of the limited options available to an administration already stretched by the crisis in Ukraine and faced with a quandary over how to deter China without damaging broader ties with the world's second-biggest economy.

The United States has refused to take sides in maritime disputes and urged negotiations. Washington itself has ruled out mediating between Beijing and Hanoi.

Some of America's friends in the region have little patience with a measured U.S. approach, especially after Obama, during his four-country Asian tour, raised expectations for an increased effort to keep China from overstepping in maritime disputes with its neighbors.

An Asian diplomat in Washington said it was essential the United States took a firmer line with Beijing while also using its influence to calm the mood in Vietnam, where anti-China rioting subsided on Friday.

He said the concern among Southeast Asian countries was that China was seeking incremental gains in provoking a series of crises with its neighbors, a tactic that could eventually change the regional landscape unless it was met with a resolute response.

ASIA ALLIES KEEP CLOSE EYE ON U.S. RESPONSE

The United States does not have a treaty obligation to defend Vietnam as it does the Philippines, and there was no sign Washington was considering tougher actions such as sending more navy ships to the area or imposing sanctions on Beijing.

Due to the intertwined nature of the U.S. and Chinese economies, there is little appetite in Washington for the kind of punitive measures that have been used against Russia over the Ukraine crisis.

Some China watchers believe a firmer U.S. stand is needed.

"The United States should be prepared to offer support to Vietnam through an increased naval presence," wrote Elizabeth Economy and Michael Levi of the Council on Foreign Relations, a well-known think tank. "This would give Washington the ability to assess Chinese capabilities and to help de-escalate the situation."

Another option, they said, could be restrictions on the U.S. activities of China's state-run oil company CNOOC, which owns the $1 billion rig.

"If the United States can't back up its words with actions, its credibility in promising to uphold peace and stability in the region will be gutted," they wrote.

The U.S. administration had hoped China would heed condemnation of its air defense identification zone in the East China Sea, which China established last year to enforce its claims on tiny Japanese-administered islands there.

But Washington now believes that Beijing learned the "wrong lesson" from Russia's annexation of Ukraine's Crimea peninsula on how to pursue its own territorial claims, a senior U.S. official said.

The official insisted that Beijing's behavior had only reinforced allies' desire for an expanded U.S. diplomatic, military and economic presence in Asia, Obama's signature second-term foreign policy initiative.

Indonesian Foreign Minister Marty Natalegawa suggested that his country might be able to help mediate the China-Vietnam standoff. "If no one tries to make some kind of an effort, then the risk of escalation and a worsening situation is a very real one," he said.